Don't show the Cypress error page on network errors

[flotwig]

When a request is proxied through Cypress and there is a network error, currently we send a valid HTML response with a 500 status code:

"Cypress errored attempting to make an http request to this url..."

This causes issues when users expect their applications to behave like the real world.

Current issues include:

• Parse Error on DELETE request #1398 - JSON being received as HTML on net error
• fix(Proxy): Fixes Can't set headers after they are sent. error thrown #957 - Cypress tries to inject the HTML mid-response-stream

If we don't send that error anymore, then there's no issue. We should just kill the browser-facing socket and let users handle the network error in application code.

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We should still retain the old error for failed file requests, since we stand up a web server for them and can serve 404's or what have you:

Cypress errored trying to serve this file from your system...

[cypress-bot]

The code for this is done in cypress-io/cypress#4105, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.3.0.